ARTESIAN WELLS. 411 



the drill; the general introversal dip assures a flow through the per- 

 meable feeds from periphery t<> center, and the low degree of dip is most 

 favorable to an extended and broad area of absorption in the bell within 

 which the strata of the upper water zone lie exposed at the surface. 



The absorbent area of the rocks of the lower water zon< — the Fox 

 Hills and lower Laramie — is reduced to a minimum, these strata being 

 vertical and exposed for absorption only along the western edge of the 

 held. In the northwestern portion of the field the advantage that other- 

 wise might have been derived from their approximately horizontal position 

 and broad area of exposure is completely counteracted by the system of 

 faults so extensively developed. The permeable strata that may occur in 

 the upper, clayey member of the Laramie present a minimum surface 

 of absorption along the foothills, but in the northwestern portion of the 

 held this surface is largely increased by the gentler dip and more extended 

 area of exposure. It is doubtful whether the body of Laramie outcropping 

 in the eastern portion of the held enters into the supply of artesian water 

 in a materia] degree, for while its dip is often to the west, much of the 

 exposed series of rocks is of a considerably higher horizon than the strata 

 underlying the developed artesian area. 



The Arapahoe formation is the most favorably exposed of all for the 

 reception of water falling upon it. With the exception of the short distance 

 from Bear to Ralston creeks, along which it is highly upturned and confined 

 in a narrow belt between the Denver and Laramie formations, the Arapahoe 

 for the most part lies at a low angle of dip, and in the broad areas over 

 which it is without cover its water-bearing beds are exposed to the falling 

 rains and become completely saturated from summit to base. 



The absorbing surface of the Denver formation is greater than that of 

 any of the other terranes, but <>n account of the comparatively slight depth 

 to which it underlies the prairies, its lack often of a confining cover, and its 

 proximity to the irregular floor of Arapahoe and Laramie beds upon which 

 it was deposited, it will not become an important factor in the artesian 

 supply of the held. 



The Quaternary of the Denver area, excepting m the stream bottoms, 

 consists largely of a porous loess, often sandy rather than argillaceous. It 

 varies greatly in depth and is frequently absent over large tracts Its value, 



